1. Field of the Invention
The present invention relates to a method of controlling oxygen deposition in decarburization annealing lines on anisotropic electromagnetic steel sheets, and more particularly relates to a method for precisely applying a target oxygen deposition value to a steel sheet.
2. Description of the Related Art
The purpose of a decarburization annealing process for anisotropic electromagnetic steel sheets is to decarburize steel sheets to prevent magnetic aging of final products, and also form an oxide film (mainly consisting of SiO.sub.2 and Fe.sub.2 SiO.sub.4) necessary for producing a vitreous film during finish annealing that is then conducted under application of an annealing separation agent. Oxide film greatly influences the behavior of secondary recrystallization during final annealing. Hence, oxide film affects magnetic characteristics of the final product and greatly influences the quality of the vitreous film formed on the surface of the final product. Therefore, the amount of oxide film produced must be strictly controlled.
Oxygen deposition is usually employed as a component of the total amount of the oxide film produced. Several methods for properly controlling oxygen deposition have heretofore been proposed. For example, Japanese Patent Laid-Open No. 3-122221 proposes a method by which components of atmospheric gas on the delivery side of a decarburization annealing furnace are compared with those on the supply side, and the amount of H.sub.2 O in the supply gas is changed in accordance with the measured difference. Also, Japanese Patent Laid-Open No. 4-337003 proposes a method by which an estimated value of oxygen deposition is determined in a regression manner based on the dew point in an annealing furnace, the sheet temperature and the soaking time, then the dew point of atmospheric gas is changed so that the estimated value coincides with a target value.
The above-described prior art methods, however, require monitoring either atmospheric gas or sheet temperature to maintain oxygen deposition at a predetermined value, which creates response lags lasting several tens of minutes or more. Accordingly, none of the prior art methods provide a quick system response necessary to maintain constant oxygen deposition during abrupt changes in the surface roughness of the steel sheet, variations in cold rolling conditions of the preceding step, etc. Additionally, long lag times exist in the prior art between implementation of corrective measures and re-establishment of the target oxygen deposition value.